The technique named SCR (Selective Catalytic Reduction) is well established for industrial plant combustion gases, and may be broadly described as passing a hot exhaust gas over a catalyst in the presence of a nitrogenous reductant, especially ammonia or urea. This is effective to reduce the NOx content of the exhaust gases by about 20-25% at about 250.degree. C., or possibly rather higher using a platinum catalyst, although platinum catalysts tend to oxidise NH3 to NOx during higher temperature operation. We believe that SCR systems have been proposed for NOx reduction for vehicle engine exhausts, especially large or heavy duty diesel engines, but this does require on-board storage of such reductants, and is not believed to have met with commercial acceptability at this time.
We believe that if there could be a significant improvement in performance of SCR systems, they would find wider usage and may be introduced into vehicular applications. It is an aim of the present invention to improve significantly the conversion of NOx in a SCR system, and to improve the control of other pollutants using a SCR system.